Telephone system



. June 24-, 1941. HQVLAND 2,247,055 7 TELEPHONE SYSTEM Filed Nov. 28',1939 3 Sheets-Sheet 1 I N 5 N TOR H. HOl LAND June 24, 1941. H, HQVLAND2,247,055

' TELEPHONE SYSTEM Filed Nov. 28, 1939 s Sheets-Sheet 2 PBX FIG. 2

IHI-

' INVENTOR H HOVLAND Patented June 24, 1941 car o STATES PATENT orrlcsTELEPHONE SYSTEM Henry Hovland, Williston Park, N. Y., ass'ignor to BellTelephone Laboratories, -Incorporated,'New York, N. Y., a corporation ofNew York K 7 Application November 28, 1939, Serial No. 306,470

14 Claims.

This invention relates to telephone systems and particularly to systemscomprising subscriberof trunks therebetween, the assignment of-fewersubscribers numbers to the private branch exchange than there are trunksin the group, and a reduction in the number of level hunting connectorswitches needed for connecting with trunks to the private branchexchange.

The invention is a trunking arrangement in which each trunk of a groupbetween a central office and a private branch exchange is connected to aset of terminals in the bank of a connector switch and to a set ofterminals in the bank of a selector switch, the selector switch beingdirectively responsive to the tens digit of the number assigned to theprivate branch exchange. Each connector switch having access to thetrunks to the private branch exchange is multiplied to terminals in thebanks of selector switches which are reached by the hundreds digit ofthe private branch exchange number and which are directively responsiveto the tens digit of this number. In the level of these selectors whichcorresponds to the tens digit of a private branch exchange number, thefirst choice terminals, for instance, the first five sets of terminals,are connected directly to five trunks to the private branch exchange;and the last choice terminals, for instance, the last five sets ofterminals, are connected to five of the connectors having access to thetrunks to the private branch exchange.

A feature of the invention is the provision of selector switchescorresponding. to the hundreds digit of numbers assigned to one or moreprivate branch exchanges, each of these selectors having direct accessin the level corresponding to, the

tens digit of a private branch exchange number to some of the trunks tothe private branch exchange and also having access to connector switcheseach of which has access to all of the trunks to the private branchexchange. If there are a plurality of private branch exchanges eachhaving a group of connector switches individual thereto, each level ofthe aforesaid selector switches corresponds to the tens digit of, and

gives access to, one of the private branch ex-' changes. The firstchoice terminals, for in,-

stance, the first five sets of terminals in each level, are connecteddirectly to trunks to the private branch exchange; and each of the lastchoice terminals, for instance, the last five sets of terminals, areconnected to'one of the connectors having access to all of the trunks tothe private branchexchange. The selectors are divided into two or moregroups, the selectors in each group-having access to some or all of theconnectors associated with each of the private branch exchanges.

Another feature of the invention is a trunk circuit associated with thecentral office end of a two-way trunk to a private branch exchange, thetrunk circuit being connected to terminals in the bflI'lkS'Ofconnectors, to terminals in the banks of selectors, and to terminals inthe banks of line finders. A relay is provided for protecting the trunkagainst double connections, a low resistance winding being normallyconnected betweenv the test terminals of the connectors and the testterminals of the selectors and an operating winding being normallyconnected to the test terminals of the connectors. The connection ofground to the test terminal in the bank of any one of the selectorsimmediately guards the trunk againstseizure by a connector, and theconnection of ground to thetest terminal in the bank of any one of theconnectors immediately guards thetrunk again'stseizure by a selector. ofseizure of the trunk of a selector, the windings of the-relay aredifferentially energized in series but the-relay does not operate; itseized by a connector, the energization of the operating winding effectsimmediate operation so that, if the trunk is simultaneously seized by aselector, the call from the connector is completed but a busy toneistransmitted to the other calling subscriber. A line circuit isassociated with each trunk to the branch exchange, the cut-off relaybeing normally connected in parallel with the operating winding of thedifferential relay of the associated trunk circuit, the operation of theline relay on a call originating in the branch exchange being effectiveto connect the operating winding of the differential relay to thetestterminal of the trunk in the banks of the line finders.

These and other features of the invention are further described bydisclosure of a system in which the invention is embodied, one suchsystem being shown schematically in the drawings which form-a part ofthis specification; The invention is, of course, not limited in itsapplication to the particular system shown nor to switches of thetypeshown but generally an- In case plicable to all automatic switchingsystems comprising an automatic central office and a private branchexchange.

Referring to the drawings:

Fig. 1 represents a central office CO comprising a subscribers line Li,line finder switches LF| and LF-Z, selector switches S|, S-2 and 8-3,connector switches C| and LHC, a line circuit LC and a trunk circuit T-Jassociated with a trunk outgoing to a private branch exchange;

Fig. 2 shows a private branch exchange PBX comprising a trunk circuitT--2 associated With a trunk to the central ofiice, a line-finder switchLF, a selector S, a connector C and acord ciri cuit CD at an attendantsposition; and

Fig. 3 shows the manner in which ten trunk circuits between a centralofiice and a private branch exchange are multipled to terminalsin thebanks of both selectors and level hunting connectors.

The line finder, selector and connector switches are of thewell-knowntwo-motion step-by-step type and reference'may' be had topages 53. to 65 of' the second edition of Automatic Telephony by Smithand Campbell for a description of the structure ofsuch switches andtheir operation when used as selectors or connectors. Reference may behad to the R. L.- Stokely Patent No. 1,799,654, April '7, 1931,for-disclosure of a twomotion step-by-step switch arranged for use as aline finder. Each of the switches LF -i, LF2, LF, SI,' CI and LHCare'represented in the drawings bya set of brushes and a singleset ofterminals. The selector S2'is represented ing scheme and thetotal'number of subscribers.-

The selector S-3 representsthird selectors in an office in' which"onlyfirst selectors, second selectors and connectors-are needed-forcomplet-- ing connections between callingand called lines,

the'thirdselectors being-provided for giving directaccess to trunks"leading to'private branch exchanges which are large enough to requirethe use of level hunting connectors in which the entire bank serves asingle private branch exchange;- An idle third selector S3 is seized inresponse to the dialing, ofthe hundreds digit of the number of "any oneof as many as ten private branch exchanges. This selector is directivelyoperated'infresponse to the tens digit of the called number to select alevel in which the first choi'ce'sets of terminals, such as H, areconnected directly to trunks leading to the private branch exchang andthelast choice sets of terminals such as l2 are'connected to levelhunting connectors L'HC which have access to all of the trunks leadingto the called private branch exchange. Each of the level huntingconnectors, which may be similar to that disclosed in the patent to H.Hovland No. 1,747,169 of February 18, 1930, are accessible to some orall of the third selectors 8-3. These selectors are divided into groups,each group having access to certain of thelevel hunting connectors as ismore particularly illustrated in Fig. '3. In' this figure there are tentrunkcircuits, five of which connect to the first five sets'of terminalsin a level of the banks of a first subgroup of third selectors, theother five trunks being connected to th first five sets of terminals inthe same level of the banks or a second subgroup of third selectors.Each of the ten trunks is also connected to a set of terminals in thebanks of all of the level hunting connectors, five of which are shown inthis figure. Each of the selectors in the first subgroup have firstchoice access to the trunk circuits T-! l to Tl5 and second choiceaccess to the five level hunting connectors LHC-l to LHC-5, and each ofthe selectors in the second subgroup have first choice access to thefive trunk circuits Tl6 to T-I0 and second choice access to the samefive level hunting connectors LHC| to LHC'5.

v If more than fiv connectors are required, they may be divided intogroups, each group accessible to some of the third selectors.

The trunks between the central ofiice and the private-branch exchangemay be two-way trunks and the trunk circuits'T'l and T2 shown in thedrawings are arranged for two-way service. On a call from a centraloffice subscriber, the trunk T| may be seized eith r by a selector S3o'r by a level hunting-connector LHC; and, in either case, seizureeffects-the lighting'of an answering lamp AL associated with'a jack J atthe attendants position in the private branch exchange. On acall fromthe private branch exchange to the central oflice' the trunk circuit T2may be seized-eitherby the insertion of the plug of an operators'cord-CD in jack J or by the selection of this trunk circuit by a selector Sin response to the dialing of the first digit at a calling station inthe private branch exchange. Theseizure 'of'the trunk circuit T-2effects the operation of the line circuit LC in the central officethereby connecting the trunk through a line finder LF2 to an associatedfirst selector, not shown in the drawings; and the connectionis'completed by selector and connector switches inresponse to thedialing of the number of the central ofiice subscriber.

Assume first that the subscriber A in the central office C0 calls aprivate branch exchange accessible to the'selector S-'3.' In'usual andwellknownmanner, the callingline L--I' is connected through a linefinder switch LF-I to a first selector S-l which isdirectively operatedby the dialing of the first digit of the number of the private branchexchange to select a corresponding" level and automatically operated toseize a set of terminals therein connected to an idle secondselectors-4. The-second selector S-Z responds to-the'second digitdialed, which is the hundreds digit, toselect a corresponding level anda set of terminals therein leading to an idle third selector S-3. Theseized selector S3 is directively operated in response'to the tens digitof the called subscribersnumber to select a correspondinglevel andt'oa'utomatically hunt ther in, Assume that one of the first'choice setsof terminals, such as H, is idle and is seized by. the selectors-landthat this set of terminals is connected to the trunk circuit T- I. Uponseizure ofterminals H, a circuit is closed for operating the line relay2| of trunk circuit T!. This circuit is traced'from battery through theupper winding of relay 2|, aback contact of relay 24 conductor'M,through the middle brush of each of switches S 3, S2, S! and LF'I,through the calling subscribers line and station, back through the upperbrushes of each of switches LF--I, S-'-l, S-'2 and 3-3, conductor 3, aback contact of relay, 24,,and throughthe lower winding of relay 2! toground: :Relay 2| closes a circuit for operating relay '22.. Relay-i22also connects ground through its inner upper front contact, the upperback contact of relay 3|, the outer lower front contact of relay 22 inparallel with the lower back contact ofrelay 23,'upper.

winding of relay 29, inner lower backcontact of relay 38, thence throughthe lower winding of relay 29 to battery. Relay 29 does not operatebecause its upper and lower windings areoppo-i sitely energized. Theinner lower back .contact of relay 35 is also connected through theinner upper back contact of relay 35, and through conductor 49 to thetest terminals in the banks-of level hunting connectors having access totrunk circuit T--l.

connected to these test terminals is effective :to guard against seizureoftrunk circuit T l ,by any of these connectors.

The aforementioned operation of relay 2| also: closes a circuit foroperatingrelay 35; andrelay 25 looks to ground at the back contact ofrelay private branch exchange, the winding of relay,

53 being normally bridged across the conductors of trunk T. Relay 53closes a circuit foroperating relay 56. Relay 56 closes a circuit foroperating relay 51. Relay 5'! connects ground to conductor T3, to markthe trunk circuit T- -2 busy; in

the banks of all selectorsS having access to this; trunk circuit, andconnects battery through re-u sistor 50 to the sleeve of jacks J. tomarkthe trunk circuit T2 busy at each of these jacks.v

When the calling subscriber dials the next digit of the private branchexchange number, relay 2| is alternately released and reoperated inresponse to each of the impulses in' the train. Relay 22 is,- slow torelease and remains operated during the response of relay 2i to -dialimpulsesr The release of relay 2!, in response to thefirst impulse,

closes a circuit for operating relay-23. Relay 23 is a slow-to-releaserelay which remains operated until all of the impulses in the trainhavebeen received, Relay 23 closes a circuit, including, its outer lowerfront contact and an upper back contact of relay 3%, for operativelyenergizing the winding of relay 28. Relay23 closes a locking circuitthrough its inner lowerfront contact toground at a front contact ofrelay 22. At its middle upper front contact, relay 28 connects ground toconductor 46, operatively energizing the winding of cut-off relay 42 ofline circuit LC. The connection of ground to conductor 45 shortcircuitsthe upper winding of relay 29 and maintains the energization of thelower winding of relay 29. Relay 2% is thereupon operated by its lowerwinding, closing a circuit for operating relay 25. Relay closes acircuit for operating relay and closes a short circuit around thewinding of relay 21. Relay 30 opens the operating circuit of relay 28,closes a connection from ground through the middle upper front contactof relay 2%, conductor 46, the inner lower front contact of relay 25,upper front contact of relay 2!], to conductor 49 to continue themarking of the trunk circuit T-l as busy in thebanksof the level huntingconnectors. Relay 3flalso opens Since the upper winding of relay 29 is alow resistance winding, the potential thus the connection betweenthe'two windings of relay 29;and connect ground through its innerlowerfront contact, the lower back contact of relay 23 in parallel with theouter lower front contact of relay 22, and through the upper backcontact of relay 3!, to conductor l5. The Ope on of relay 30 also causesthe release of relay 35; and the release of relay 35 disconnects batteryand ground from the conductors of trunk T, releasing relay 53. Relay 53opens the operating circuit of relay 52 but relay 56 is slow inreleasing and, before it can release, another operating circuit isclosed by the operation of relay 54 as hereinafter described, With relay28 operated and relay 23 released, a circuit is closed from thegrounded, ring-- ing current generator 32, through the inner upper frontcontact of relay 2%, lower winding of relay 26, upperback contact ofrelay 23, lower back contact of relay 26, lowermost front contact ofrelay 28, lower conductor of trunk T, conductor 52 of trunk circuitT.-2,'windin g of relay 53in parallel with winding of relay 54 andcondenser 55 in series, through upper back contacts of relay 61, anupper back contact of relay 52, conductor 5|, the upper conductorof'trunk T, uppermost front contact of relay 2%, to ground at a backcontact of relay 25. While this ringing circuit is closed, ringing toneis transmitted through condensers land is and-conductor M to the callingsubscriber. The ringing current operatively energizes the winding ofrelay 5d and may cause relay 53 to vibrate. Relay 53 will be operated bydirect current from the tripping battery during the silent part of theringing cycle. Relay 54 closes a circuit through the winding of relay 56and so also does relay 53. Relay 56 maintains the circuit for operatingrelay 5'5. With relays 5t and 51 both operated, a circuit is closed foroperating relay 58; and relay 58 closes a circuit for lighting theanswering lamp AL. Relay 58 also closes a circuit for operating relay52; and relay 59 closes a circuit through the lower back contact ofrelay 68 for locking relay 58.

When an attendant at the private branch exchange answers the call byinserting the plug of a cord CD in jack J, a circuit is closed at thejack for holding relay 51 operated; and a circuit is closed through thetip and ring springs of the jack, middle lower front contact of relay 51and outer back contacts of relay 6'? for operating relay 6|. Relay 6|closes a holding circuit for relay 56, closes a circuit for operatingrelay 62, and connects ground toconductor 1%. Relay 52, which is slow inoperating, opens the shunt throughresistor 64 across the winding ofrelay 65, and completes a circuit path from conductor 5| through thewinding of relay 55, inner upper and outer lower front contacts of relay62, to conductor 52. The winding of relay 55 is thus bridged across theconductors of trunk T and ground is connected through resistor. 69 and alower front contact of relay 52 to conductor 52 to cause an increasein'the current through the lower, tripping winding of relay 26 of trunkcir cuit Tl. Relay 62 also closes a circuit for operating relay 53.Relay 63 locks independent of relay 58 and connects the winding of relay65 across conductors 5i and 52 independent of relay 62; Relay 25 isoperated suficiently to close its inner upper front contact; and theenergization of its upper, locking winding completes the operation ofrelay 25. Relay 26 opens the ringing circuit and connects relay 24to'the conductors of trunk T; and relays 24 and 65,

operate in series through'resistor 69 to ground at relay 56.. Relay 65closes a circuit for operating relay 56; and relay 66 disconnects thetripping ground from conductors 5| and 52 and closes circuits foroperating relays 61 and 68.

Relay 6? closes a circuit for holding relay 56 operated and opens thecircuits through the windings of relays 6| and 53. Relay 6| releasesrelay 62, reconnecting resistor 64 in shunt with denser 55 remainbridged across the trunk. Withrelay 26 of trunk circuit T-| and relay 61of trunk circuit T2 both operated, a talking con-' nection is completedfrom the calling line through condensers I! and I8 of trunk circuit TI,the outer front contacts of relays 26 and 28, the conductors of trunk T,conductors 5| and 52 of trunk circuit T2, winding'of relay 65, outerfront contacts of relay 6?, a front contact of relay 51, through the tipand ring co'n-' ductors of jack J and plug of cord CD, to the operatorstelephone set.

When the calling subscriber releases the connection, relay 2| releases,closing the circuit for' operating relay 23. The continued release ofrelay 2| causes the release of relay 22. Relay 22 disconnects groundfrom conductor I5 thereby effecting the return to normal of switchesLF-l, S|, S2 and S3 in usual and wellknown manner. The release of relay22 also effects the release of relays 23, 26 and 28. The release ofrelay 26 disconnects relay 24- from the trunk thus releasing relay 65 oftrunk circuit T2 and causing a disconnect signal in the operators cordcircuit CD. The release of relay 65 causes the release of relay 66. Therelease of relay 66 causes the release of relays 61, 68 and 56. theoperator disconnects the cord from jack J.

When the plug is removed from jack J, relay 51 releases and the busyground is disconnected from conductor 13. The release of relay 28 oftrunk circuit T-| causes the release of relays 29 and 25 in succession.The release of relay 25 opens the energizing circuit of theslow-torelease relay 30. branch exchange has released the trunk inresponse to the disconnect signal, relay 3!] releases and the trunkcincllit T2 is in normal condi tion ready for use on another call. 7

If the operator at the privatebranch exchange fails to remove the plugof cord CD from jack J in response to the disconnect signal, relay 6|reoperates when relay 6! releases; and relay 6| holds relay 56 operatedand reoperates relay 62.

Relay 6| connects ground potential to conductor 13 to prevent seizure ofthe trunk circuit T2, bridges the winding of relay 65 across conductors5| and 52, and also connects ground at the back contact of relay 66through resistor 69 and conductor 52 to the lower conductor of trunk T;

thereby operating the line relay 4| of the line circuit LC associatedwith trunk T in the central c-fiice. Relay 65' does not reo-perate sincethe upper conductor of trunk T is not connected to ground in the centralofiice. I trunk circuit Tl operates in series withthe line relay 4|, theshort circuit across the'w'ind ing of relay 21 having been opened bytherelease of relay 25.

release of relay 25 does not cause' the release Relay 51 is held toground at jack J until" If the operator at the private Relay 21 of Theoperation of relay 27- closes a holding circuit for relay 30-'so' thatthe of relay 30. Withrelay 30 operated, ground potential is connectedthrough its inner lower front contact, lower back contact of relay 23,and upper back contact of relay 3| to conductor |5 to prevent seizure oftrunk circuit T2 by any of the selectors S3, and ground potential isconnected through the lower back contact of relay 25 and upper frontcontact of relay 36 to conductor 49 to prevent seizure of trunk circuitT2 by any .ofthe. level hunting connectors LHC. The line relay 4|connects the Winding of the cut-off relay 42, .throughconductor 43 totest terminals in the banks of line-finder switches LF-2 having accessto the trunk T. Relay 4| also closes a circuit .for operating a grouprelay 46 and relay ,4'0 initiates the operation of an idle one of theline. finders to connect the trunk T to a first selector in usual andWell-known manner. When a line finder finds the terminals of the callingtrunk, ground is connected to the test terminal in usual manner toefiect the operation of the cut-off relay 42;.and the operation of relay42 causes the release of relays 4| and 46. The connection of ground tothe test terminal also causes the energization of the operating windingoflrelay 29a Relay 29 closes the circuit for operating relay 25; andrelay 25 closes the circuit for operating'relay 30 and closes the shortcircuit around the winding of relay 21. Relay 30 continues to connectbusy ground to conductorl5- and connects the test terminal of the linefinder to the test terminals in the banks of the connectors. The trunk Tand associated trunk circuits are thus held until the attendant at theprivate branch-exchange removes the plug and a permanent signal alarmmay be operated in the central oiiice; When the plug is removed,thereleas'e of relays 6| and 62 causes the release of relays 56 and 5'!in succession. The release of relay 51 disconnects guarding potentialirom conductor 13' and from jacks J. Relay 62- disconnects 'resistor 69from conductors 5| and 52 thereby causing the release of the firstselector and line-finder switches in the central oifice. 'When ground isdisconnected from the test terminal of the line finder, relays 42 and 29release; 'The release of relay 29 causes the successive release ofrelays 25 and 30 thereby disconnecting the busy ground potential fromconductors l5 and 49. The trunk T is now free for seizure at either end.I

If a selector S'3 is operated to select the level including trunks T toa private branch exchange and all 'of the first choice sets of terminalsare busy, the brushes continue to hunt over the second choice sets ofterminals until an idle level hunting connector LHC is found. Theselected level hunting connector receives the last digit ialed' by thecalling subscriber and is operated in well-known manner to select anidle trunk T to the private branch exchange. Upon seizure oftru'nk T bya level hunting connector LHC, groundpotential is connected, in usualmanner, through its test brush to conductor 49 to guard against seizureby any other level hunting connectorjand is further connected throughthe upper low're'sistance winding of relay 29 to conductor |5 to guardagainst seizure of the trunk by any of the selectors. This groundconnection also causesthe operation of cut-off relay 42 and causes thesuccessive operation of relays 29, 25 and 3D. Relay 42 prevents theoperation of line relay 4| Relay 30 connects ground potential directlytoconductor l5 as hereinbefore described toprevent' seizure of trunkcircuit Tl by any of the selectors S-3. The connector LHC transmitsringing current over trunk T; and the trunk circuit T2 operates ashereinbefore described to light the answering lamp AL and mark the trunkcircuit T2 busy. When the call is answered by an attendant at theprivate branch exchange, the trunk circuit T-2 functions as hereinbeforedescribed and the talking connection is completed by the level huntingconnector in usual manner. When the calling subscriber releases theconnection, the connector and preceding switches in the central officeare returned to normal and the trunk circuits T-l and T'-2 are restoredto normal.

If, on a call from the private branch exchange to the central ofilce,the trunk circuit T-2 is seized by an attendant, the insertion of a plugin jack J causes the operation of relays 51' and 6|. If the trunkcircuit T-2 is seized by a selector S in the private branch exchange,relay El operatesin serieswith the calling extension. In; either case.relay 6| closes .circuits for operating relays 55 and 62 and connectsground potential to conductor 13 to prevent seizure of trunk circuit T2by a selectorIS. 'Relay'56 closes a circuit for operating relay 51. andrelay 51 connects busy. potential'tothe sleeves of jacks J. With yrelayE2 operated, ground is connected through resistor E9'to conductor. 52 toeffect the operat on of the line relay 4| in the central'office andextension of the trunk through a line finder to a first selector. Relay2'! of trunk circuit T.--! is operated in series with relay H, closingthe circuit for operating relayBll. Relay 39 connectsbusy-marking,ground potential to conductors l5 and 49 as above described. When a l nefinder finds the terminals of trunk T, ground is connected through thetest brush, operating the cut-off relay 427to release relays 40 and 4!.Relay 42 also extends this ground potential through its left frontcontact toconductor 46 and the lower winding of relay 29. Relay 28operates closing the circuitfor operating relay 25 and relay25-short-circuits relay 21. With relays 25 and Sllyboth operated, the

ground connected through the test brush of the line finder LF-fZ toconductor 46 is extended through conductor 49-to the test terminals-inthe banks of the level hunting connectors. Upon connection of trunk Tthrough the brushes of a l ne finder LF--2 to the line-relay.(nct'shown) of an associated first selector, relay B5 of trunk circuitT2 is operated Relay, 65 closes the circuit for o erating relay 66.:Relay 65 disconnects' ground from conductors 5i and 52 and loses thecircuits for operating relays 1i! and 6%. Relay 6'! connects theconductors of the trunk to the calling operator or calling extension,causes the release of relays BI and 62. The release of relay 62reconnects resistor 64 in parallel with the winding of relay fi fi andopens the short circuit across conductors 5I and152 thereby clearing theconnectionrfor transmission of dial impulses to theswitches' inthe-central office. Relay 65 may follow dial impulses; "but, being slowin releasing, relay BG-hclds relays 61 and 68. The impulses, transmittedfrom the calling operators or subscribers dial, eifect the operation ofselector and -connectorswitches in the central office to completetheconnection. If the call is to a subscriber's line. the connection isunder the control of the calling subscriber or operator in the privatebranch exchange. If the call is to an operator. the connection is underthe joint control of the called operator and the calling subscriber oroperator, in which case release only by the calling subscriber oroperator effects the successive release of relays 65, 66, 61 and 68. Therelease of relay 8'! bridges the winding of relay 53 across the trunk tohold the switches in the central oifice. Relay 53 operates, closing acircuit for holding relay 56; and relay 56 holds relay 5'! to maintainthe busy-marking of trunk circuit T2 and hold the switches in theprivate branch exchange. When the calling subscriber again removes thereceiver or the attendant again inserts the plug of a cord, relays 6iand 62 are reoperated, relay 53 releases, relays 65, 65, 61 and 68reoperate, and relays BI and 62 again release. When the connection isreleased by the calling station or attendant in the private branchexchange and also by the called operator in the central office, the linefinder, selector and connector switches in the central office arereturned to normal. and ground is disconnected from conductor 43 torelease the cutofi relay #2. Relays 29, 25 and 30 of trunk circuit T!are thereupon released. In the private branch exchange the release ofrelays 65, 65, 6! and (-38 effects the release of relays 56 and 5'! andthe disconnection of ground potential from conductor l3 and thedisconnection of guarding potential from jacks J.

If the trunk circuit T'2 is seized at substantially the same timeby botha selector 8-3 and a level hunting connector LHC, relay 29 is op eratedby the ground connected to conductor 49 by the level hunting connector.The operation of relay 29 causes the operation of relays 25 and 3B.Relay 2! is operated in series with they line 'of the subscriberconnected to trunk circuit T2 through a selector S-3.

Relay 2| closes the circuit for operating relay 22. The call incoming totrunk circuit T--! from the level hunting connector may be completed butabusy tone signal is transmitted from tone source 20 through the innerlower front contact of relay. 2!, inner upper front contact of relay 25,lower back contact of relay 2%. condenser I3 and conductor Id to thesubscriber calling through the selector S- 3. If this subscriber dials,relay 28 cannot be operated since its operating circuit is open at theouter upper back contact of relay 3!). When the subscriber receiving thebusytone releases, relays 2! and 22 release and relay 23 is temporarilyoperated. When relay 22 releases. ground is disconnected from conductorI5. to release the selector S-3 and, preceding switches. when relay 23releases, ground is reconnected to conductor l5 to prevent seizure byanother selector.

}er. 32,.the brush of which is momentarily con- ,.nected to batteryonceevery thirty seconds. As

soon. after the release of relay 24 as the interrupter 32 connectsbattery to relay 3!, relay 3| operates and locks independent of theinterrupter.

Relay 31 connects ground to the conductor leading to the alarm. 34 andconnects ,interrupter 33 to the Winding of relay 28. As soon afterrelayit operates asthe brush of interrupter 33 is connected to ground, whichmay be eighteen seconds after the brush of interrupter 32 is connectedto battery, the winding of relay 28 is short-circuited and relay 28releases. The release of relay 2% disconnects relay 24 and talkingcondensers l1 and I 8 from the trunk thus preventing the call from theselector from interfering with the seizure of trunk T by a level huntingconnector or from interfering with calls from the private branchexchange to the central oflice. If and when the subscriber callingthrough the selector releases, relays 2| and 22 release and relay 23operates temporarily. The release of relay 22 causes the release ofrelay 26 and relay 26 causes the release of relay 3|.

Each time the trunk circuit TZ is seized by a' selector S3, themomentary operation of relay 23, when the connection is released, closesa circuit for operating the register 39.

Although in the arrangement shown in the drawing the trunks to theprivate branch ex change are connected to terminals in the banks ofselectors and level hunting connectors in the central office, theinvention is applicable to an arrangement in which the trunks areconnected to the banks of rotary connectors instead of level huntingconnectors.

What is claimed is:

1. In a telephone system, subscribers lines, a central oflice, a privatebranch exchange, a group of trunks between said oflice and exchange,selectors and connectors in said central oflice, and means forconnecting a calling line with a selector, the trunks in said groupbeing multipled to first choice sets of terminals in a level of the bankof each of said selectors and to terminals in the banks of saidconnectors, each of the remaining sets of terminals in said level of thebanks of said selectors being connected to one of said connectors.

3. In a telephone system, an automatic ofiice comprising lines,subscribers stations, a dial at each station, selectors, trunk huntingconnectors, and means for connecting a calling line with a selector, aprivate branch exchange, a group of trunks between said ofiice and saidbranch exchange, each of said trunks being connected to firstchoiceterminals in a level of the banks of selectors and to terminals inthe banks of trunk hunting connectors, each of said connectors beingconnected to other terminals in said level of the banks of selectors,means in each of said connectors for transmitting ringing current overany of said trunks, means in the branch exchange for each trunk formarking the trunk busy in the branch exchange in response to ringingcurrent from the central'office, and means for each trunk for markingthe trunk busy in the branch exchange in response to seizure of thetrunk by one of said selectors.

4. In a telephone system according to claim 3, means for each trunkeffective upon seizure of the trunk by one of said selectors for markingthe trunk busy to the other selectorsand to said connectors, and meansfor each trunk eifective upon seizure of the trunk by one of saidconnectors for marking the trunk busy to the other connectors and tosaid selectors.

5. In a telephone system, an automatic office comprising lines,subscribers stations, a dial at each station, selectors, trunk huntingconnectors, and switching means for connecting a calling line with aselector, a private branch exchange, a

group of two-way trunks between said oflice and said branch exchange,each of said trunks being connected to first choice terminals in a levelof the banks of selectors in the central office and to terminals in thebanks of said trunk hunting connectors, each of said connectors beingconnected to terminals in said level of the banks in the central officefor closing a circuit through the-trunk to" operate said circuit means,means in each trunk circuit efiective upon seizure by a selector in thecentral ofiice for absorbing the dialimpulses' thereafter created by thedialing of any remaining digits of the branch exchange number by thecalling subscriber, and means in each trunk circuit effective after theabsorption of said dial impulses 'for transmitting ringing 'curr'entover said trunk to operate the signal means associated with said trunkin the branch exchange and to maintain the marking of said tru'nk'asbusy 'in' the branch exchange.

'6. Ina telephone system, an automatic office comprising lines,subscribers stations, a dial at each station, selectors, trunk huntingconnectors, and switching means for connecting a calling line with aselector, a private branch exchange comprising selectors and a manualswitchboard, a group of two-way trunks between said office andsaidbranchexchange, each of said trunks being connected to a jack atsaid switchboard, to

terminalsinthe'banks of the selectors in the branch exchange, to firstchoice terminals in a level of 'thebanksof selectors in the centraloffice and to terminals in the banks of said trunk hunting connectors,each of said connectors being connected to te'rminals'in said level ofthe banks 'of selectors, trunk circuits in said oifice one for each ofsaid trunks, signal means for each of said trunks in said branchexchange, cords in the "branch' exchange for'connecting with said jacks,circuit means at said branch exchange for each trunk for marking thetrunk busy in the branch exchange, means in each of said trunk circuitsresponsive to seizure by any one of said selectors in the central officefor closing a circuit through the trunk to operate's'aid circuit means,means in 'ea'ch trunk circuit effective upon seizure by a selector-inthe central 'ofiice for absorbing the dial impulses thereafter'cre'atedby the dialing of any remaining digits of the branch exchange number bythe calling subscriber, and means in 'each trunk circuit effective afterthe absorption 'of said dialimpulses for transmitting ringing currentoversaid trunk to operate the signal means'associateaj with said trunkin the branch exchange and to ma'intain'the marking of said trunk asbusy in the branch exchange.

7.I'n a telephone system according to claim 6',

a source of talking current in each trunk cirf'cjuit, "me'ans in eachtrunk circuit responsive to the methane: a cord with the jack of the'triinkf'tofanswer'the call for disconnecting the r'inging'currentsource and closinga talking circuit, the closure of said talking circuitbeing e'fie'ctive to maintain the marking of said trunk as busy in thebanks of the 'selectors'in the branch exchange. 1

8. In a telephone system, an automatic central office comprising lines,subscribers stations, a dial at each station, selectors, and trunkhunting connectors, a private branch exchange, trunksbetween said officeand said exchange, each of said trunks being connected to terminals in alevel of the banks of selectors and to terminals in the banks of trunkhunting connectors; each of said connectors being connected to a-set ofterminals in said level of some of said selectors, means for connectinga calling line wasuecror, means for each of said trunks responsive toseizure by any one of said selectors for closing a signaling circuitover said trunk, means in said branch exchange for each of'said trunksfor marking the trunk busy, means in the branch exchange for each ofsaid trunks for operating the busy-marking means responsive to closureof the signaling circuit upon'seizure of the trunk by a selector, meansin said, automatic ofiice for each of said trunks for absorbingimpulses'transmitted from the dial at a calling station and forthereupon connecting a source of ringing ,cur-. rent to the trunk, acalling signal in the branch exchange for each of said trunks, means inthe branch exchange for each of said trunks responsive to ringingcurrent from the centrali office for operating the calling signal andfor operating the busy-marking means, means in the branch exchange forseizing any one of said trunks onv a call to the central ofiice,andmeans in the central ofiice for each of said trunks re sponsive toseizure of the trunkby a selector or a connector in the central officeand responsive to seizure in the branch exchange for marking the trunkbusy in the central oflice.

9. In a telephone system, an automatic central office comprising lines,subscribers stations,-a dial at each station, line finders, selectors,and trunk hunting connectors, a private branch exchange, trunks betweensaid office and said exchange, each of said trunks being connected toterminals in a level of the banks of selectors, to terminals in thebanks of trunk hunting'connectors and to terminals in the banks of linefinders, each of said connectors being connected 'to a set of terminalsin said level of some of said selectors, line circuits in said ofiice,one for each of said trunks, means for connecting a calling line to aselector, means for each of said trunks responsive to seizure by any oneof said selectors for closing a signaling circuit over said trunk, meansin said branch exchan e for each of said trimks for marking the trunkbusy, means in the branch exchange for each of said trunks foreperatingthe busy-marking means responsive to closure of the signaling circuitupon seizure of the trunk by a selector, means in said automatic ofiioefor each of said trunks for absorbing impulses transmitted from the dialat a calling station and for thereupon connecting a source of ringingcurrent to the trunk, a calling signal in the branch exchange for eachof said trunks, means in the branch exchange for each of said trunksresponsive to ringing current from the central office for operating thecalling signal and for operating the busy-marking means, means in thebranch exchange for seizing one of said trunks on a call to the centraloflice, and means in the central office for each of said trunksresponsive to seizure of the trunk by a selector or a connector in thecentral office and responsive to seizure in the branch exchange formarking theztrunk busy in the central office.

'10. In atelephone system, an automatic onice comprising lines,subscribers stations, a dial at each station, selectors, level huntingconnectors,

and means for connecting a calling line to a,

selector, a private branch exchange, trunks between said oflice and saidbranch exchange, each of said trunks being connected to first choiceterminals in a'level of the banks of some of said selectors and toterminals in the banks of all of said connectors, each of saidconnectors being connected to terminals in said level of some of saidselectors, trunk circuits in said office, one for each of said trunks,trunk circuits in said exchange, one for each of said trunks, means ineach of said trunk circuits at said office responsive to seizure by anyone of said selectors for marking said trunk busy to the other selectorsand to said connectors and for transmitting a seizure signal to thebranch exchange, means in each of said trunk circuits at said ofiiceresponsive to seizure of the trunk by any one of said connectors formarking the trunk busy to the other connectors and to said selectors,means in each of said connectors for connecting a source of ringingcurrent to a seized one of said trunks, busymarking means in each ofsaid trunk circuits at the branch exchange responsive to a seizuresignal transmitted from the trunk circuit at the central office end ofthe trunk, and means in each of said trunk circuits at the branchexchange responsive to ringing current from the central office.

4.11. In an automatic telephone ofiice comprising dial subscriberslines, selectors, trunk hunting connectors, trunks and trunk circuits,one for each of said trunks, connected to terminals in the banks ofselectors and connectors, each of said connectors being connected toterminals in the banks of some of said selectors, and means in each ofsaid trunk circuits responsive to seizure of the trunk circuit by aselector or connector for marking the trunk busy in the banks of theselectors and connectors, said last-mentioned means comprising a relayhaving a winding normally connected between the test terminals of thetrunk in the banks of said selectors and the test terminals of the trunkin the banks of said connectors, said winding having a low resistance sothat the connection of a busy-marking potential to either side of saidwinding upon seizure of the trunk by a selector or a connector marks thetrunk as busy in the banks of all of said selectors and connectors, saidrelay having another winding normally connected in series with the lowresistance windingto the test terminals of the trunk in the banks ofsaid selectors, said windings being difierentially and non-operativelyenergized upon seizure of the trunk by a selector, said other windingbeing operatively energized upon seizure of the trunk by a connector,and means controlled by the relay for continuing the busy-marking of thetrunk in the banks of the selectors and connectors.

12. In an automatic telephone onice comprising dial subscribers lines,line finders, selectors, trunk hunting connectors, trunks and trunkcircuits, one for each of said trunks, connected to terminals in thebanks of line finders, selectors and connectors, each" of saidconnectors being connected to terminals in the banks of some of saidselectors, and means in each of said trunk circuits responsive toseizure of the trunk circuit by a line finder, selector, or connectorfor marking the trunk busy in the banks of the selectors and connectors,said last-mentioned means'comprising a relay having a Winding normallyconnected between the test terminals of the trunk in the banks of saidselectors and the test terminals of the trunk in the banks of saidconnectors, said winding having a low resistance so that the connectionof a busy-marking potential to either side of said Winding upon seizureof the trunk bya line finder, selector or connector marks the trunk asbusy to all of said selectors and connectors, said relay having anotherWinding normally connected in series With the low resistance Winding tothe test terminals of the trunk in the banks of said selectors, saidwindings being differentially and non-operatively energized upon seizureof the trunk by a selector, said other Winding being operativelyenergized upon seizure of the trunk by a line finder or a connector, andmeans controlled by the relay for continuing the busymarking of thetrunk in the banks of the selectors and connectors.

13. In a telephone system, an automatic oflice, a private branchexchange, subscribers lines, a group of two-Way trunks between saidoflice and said exchange, selectors and trunk hunting connectors in saidautomatic office, said trunks being multipled to first choice terminalsin a level of the banks of said selectors and to terminals in a level ofsaid connectors, each of the other terminals in said level of theselectors being connected to one of the connectors, trunk circuits, onefor each of said trunks, means for connecting a calling line in saidautomatic oifice with an idle one of said selectors, means comprising adial at the calling subscribers station for directively operating theassociated one of said selectors to select said level, means in saidselector for seizing the first idle set of terminals in said level,means in each of said connectors responsive to a train of dial impulsesfrom a connected calling station for operating the connector to seize anidle one of said trunks, means in each of said trunk circuits effectiveupon seizure by said selector for absorbing the impulses thereaftercreated by the dialing of the remaining digits of the number of saidprivate branch exchange, means in each of said trunk circuits effectiveonly upon seizure by one of said selectors for immediately sending aseizure signal to said private branch exchange, selectors in said branchexchange, said trunks being multipled to terminals in the bank of saidselectors in the branch exchange, means in said branch exchange for eachtrunk responsive to said seizure signal from the trunk circuit in saidoflice for marking said trunk busy in the banks of said r selectors inthe branch exchange, means in each of said trunk circuits effective onlywhen seized by a selector and after receiving the dial impulses of saidremaining digits for connecting a source of ringing current to thetrunk, means in each of saidconnectors for connecting a source ofringing current to a seized one of said trunks, an operators position insaid branch exchange, signaling'means for each of said trunks at saidoperators position operatively responsive to ringing current from theautomatic office, means comprising a cord for answering the call, alarmmeans common to said trunk circuits, and means ineach of said'trunkcircuits for operating said alarm means in the event that a callingsubscriber fails to release the connection over a trunk to the branchexchange Within a predetermined interval of time after the trunk isreleased by the answering operator.

14. In atelephone system, an automatic oilice, a private branchexchange, subscribers lines, a group of two-way trunks between saidoffice and said exchange, selectors and trunk hunting connectors insaidautomatic office, said trunks being multipled to first choiceterminals in a level of the banks of said selectors and to terminals ina level of said connectors, each of the other terminals in said level ofthe selectors being connected to one of the. connectors, trunk circuits,one for eachof said trunks, means for connecting a calling line in saidautomatic ofiice with an idle one of said selectors, means comprising adial at the calling, subscriberfs station for directively operating theassociated-one of said selectors to select said level, means in saidselector for seizing the first idle set of terminals in said level,means in each of said connectors responsive to a train of dial impulsesfrom a connected calling station for operating the connector to seize anidle one of said trunks, means in each of said trunk circuits effective,upon seizure by said selector for absorbing the impulsesv thereaftercreated by the dialing of the remaining digits of the number of saidprivate branch exchange, means in each of said trunk circuits effectiveonly upon seizure by one of said selectors for immediately sending aseizure signal to said. private branch exchange, selectors in saidbranch exchange, said trunks being multipled to terminals in the bank ofsaid selectors in the branch exchange, means in said branch exchange foreach trunk responsive to said seizure signal from thetrunk circuit insaid ofiicefor marking saidtrunk busy in the banks of said selectors inthe branchexchange, means in each of said trunk circuits effective onlywhen seized by a selector andafter receiving the dial impulses of saidremaining digits for connecting a source of ringing current to thetrunk, means in each of said connectors for connecting a source ofringing current to a seized one of said trunks, an operators position insaid branch exchange, and signaling means for each of said trunks atsaid operators position operatively responsive to ringing current fromthe automatic office.

HENRY HOVLAND.

